High-speed optical image acquisition systems are used in a variety of environments to analyze the physical characteristics of one or more targets. Generally, such systems include an image acquisition system, such as a camera, that can acquire one or more images of the target. The images are then analyzed to assess the target. In many cases, the inspection is performed while there is relative motion between the target and the image acquisition system. As used herein, a “high-speed” optical image acquisition system is any system that acquires images of one or more targets while there is relative motion between the target and an image acquisition system. One particular example of such a system includes a phase profilometry inspection system such as that used in some modern solder paste inspection systems.
Phase profilometry inspection systems are currently used to inspect three-dimensional aspects of target surfaces. The concept of phase profilometry is relatively simple. A pattern or series of patterns of structured light are projected upon a target at an angle relative to the direction of an observer. This can be likened to sunlight passing through a Venetian blind and falling upon a three-dimensional object, where the object is viewed from an angle that differs from that of the sunlight. The pattern is distorted as a function of the object's shape. Knowledge of the system geometry and analysis of the distorted image or images can provide a map of the object in three dimensions.
Generally, phase profilometry systems employ a source of structured, patterned light, optics for directing the structured, patterned light onto a three-dimensional object and a sensor for sensing an image of that light as it is scattered, reflected or otherwise modified by its interaction with the three-dimensional object.
Phase profilometry inspection can be performed while there is relative motion between the target and the inspection system. This feature is highly beneficial for automated inspection machines where throughput is very important. One example of a phase profilometry inspection system is the Model SE 300 Solder Paste Inspection System available from CyberOptics Corporation of Golden Valley, Minn. This system uses phase profilometry to measure height profiles of solder paste deposited upon a circuit board prior to component placement. The SE 300 System is able to acquire the images that it uses for three-dimensional phase profilometry while allowing continuous relative motion between the sensor and the target. This is because a strobe illuminator is operated to provide short exposure times thus essentially freezing motion. Specifics of the illumination and image acquisition are set forth in greater detail in the parent application.
Solder paste, itself, presents a relatively favorable target in that it is comprised of a number of tiny solder spheres. Because there are so many small, spherical reflectors in each solder deposit, in the aggregate, each solder deposit provides a substantially diffuse optical surface that can be illuminated and imaged by a sensor configured to receive diffusely scattered light. The relatively uniform reflectivity of the solder paste deposits facilitates imaging.
High-speed inspection systems such as the surface phase profilometry inspection described above provide highly useful inspection functions without sacrificing system throughput. Theoretically, such inspection systems would be highly useful for any inspection operation that requires height information as well as two-dimensional information from a system without adversely affecting system throughput. However, there are real-world hurdles that hinder the ability to extend the advantages of high-speed inspection beyond targets having relatively diffuse reflectivity such as solder paste.
Surfaces such as the underside of a ball grid array (BGA) or chip scale packages (CSP's) are difficult to inspect using current optical phase profilometry systems. Specifically, the balls on a BGA are constructed from reheated solder, rendering them substantially hemispherical and shiny. Illuminating the hemispherical shiny balls with a structured or directional light source will generate a bright glint from a portion of the surface near the specular angle. Conversely, there will be nearly no light returned by diffuse scattering from the remainder of the hemispherical surface. Imaging the ball with a sensor that is not configured to deal with this wide dynamic range of returned light levels would result in erroneous data.
When light falls upon any surface, some light will be specularly reflected and some will be diffusely scattered. Some surfaces (like a mirror) are predominantly specular in their reflections, and some surfaces (like a piece of paper) predominantly scatter light. Imaging any such surface, even highly specular surfaces, may be possible by extending the dynamic range of a sensor configured to receive diffusely scattered light. One way in which dynamic range has been extended in the past is by taking or acquiring multiple images of a target with different illumination levels and processing the images to discard image data such as saturated pixels or dark pixels. However, all dynamic range extension techniques currently known are believed to be applied solely to systems in which the target and inspection system do not move relative to each other while the multiple images are acquired. Thus, as defined herein, such systems are not “high-speed.” In inspection systems where throughput cannot be sacrificed by pausing relative movement between the inspection system and the target, dynamic range extension itself is not easy to implement.